1. Field of the Invention
The present invention is generally related to a geartooth sensor and, more particularly, to a geartooth sensor that measures the magnetic field strength imposed on a magnetically sensitive component and compares the magnetic field strength to a threshold that is calculated as a function of the minimum magnetic field strength and average magnetic field strength over a preselected period of time.
2. Description of the Prior Art
Many types of geartooth sensors are known to those skilled in the art. In order to detect the presence of a geartooth within a detection zone of the geartooth sensor, a threshold value is first determined and subsequent signals received from a magnetically sensitive component are compared to the threshold value to determine the presence or absence of a ferromagnetic geartooth within the detection zone.
In order to determine the appropriate threshold with which subsequent signals are compared, certain known geartooth sensor circuits determine a minimum value of the output signal from a magnetically sensitive component and then add a fixed offset magnitude to that minimum value. For example, if the minimum value is calculated to be 2,000 gauss, a fixed offset magnitude of 80 gauss can be added to the minimum value to create a threshold magnitude of 2,080 gauss. Subsequent signals from the magnetically sensitive component would then be compared to this threshold magnitude to determine the presence or absence of a geartooth within the detection zone. Other geartooth sensors determine the average value of the output signal from the magnetically sensitive component over a preselected period of time and use that average value as the threshold magnitude with which subsequent output signals from the magnetically sensitive component are compared.
U.S. Pat. No. 5,164,668, which issued to Alfors on Nov. 17, 1992, discloses an angular position sensor with decreased sensitivity to shaft position variability. The sensor is provided with first and second pole pieces that extend from regions proximate a rotatable magnet to regions proximate a magnetically sensitive device. The pole pieces provide magnetic paths of lowered reluctance that confine the lines of flux extending between the rotatable magnet and the magnetically sensitive device. The placement of the rotatable magnet between first and third pole piece segments of the invention significantly reduces the sensitivity of the sensor to variations in position of the rotatable magnet and therefore increases the reliability of the measurement system. This reduced sensitivity inhibits the degradation of operation accuracy that could otherwise be caused by inaccuracies in the magnet's shaft position, large tolerances in the dimensions of the shaft diameter and the bearing diameter and variable location of the shaft because of excessive bearing wear.
U.S. Pat. No. 4,992,731, which issued to Lorenzen on Feb. 12, 1991, describes a rotary speed sensor with baseline compensation of Hall cell output signal. The sensor system uses a permanent magnet and a Hall cell which is sensitive to the changing tangential component of magnetic field caused by the interrupted surface profile of a rotary element. In order to avoid anomalies caused by the variation of the base value of the tangential component of magnetic field, the output of a differential amplifier fed by the Hall cell is connected to a voltage averaging circuit which stores the average voltage of the output on a single capacitor. The output of the differential amplifier and the voltage across the capacitor are respectively coupled to the differential inputs of a Schmitt trigger exhibiting hysteresis. The Schmitt trigger thereby establishes operate and release points relative to the average voltage signal. The voltage averaging circuit is a nonlinear circuit in which the voltage across the capacitor is fed back to the negative input of an operational amplifier. A power up circuit is also provided for rapidly charging the capacitor initially to approximately the average voltage.
An SAE Technical Paper titled "The Emergence of a New Sensor for Ignition Timing" by Mark P. Podemski was presented at the International Congress and Exposition in Detroit, Mich. in February 1987. The paper describes sensors which are used for ignition timing and examines a new sensor that is based upon a technology which has been proven in the automotive market. Hall effect vane sensors have been used for many years. This paper describes the development of a Hall effect probe sensor that was developed to improve performance and reduce costs. Traditionally, Hall effect sensors have required either a magnet or fabricated vane targets to be mounted on a rotating shaft in order to receive appropriate position information about a rotatable member. The probe sensor described in this paper utilizes a specially designed integrated circuit to sense changes in magnetic flux densities created by the proximity of a metal target such as a geartooth. The sensor was designed particularly for the automotive market.
In a product brochure titled "Hall Effect Geartooth Sensors (GT1 Series)" geartooth sensors are described which use magnetically biased Hall effect integrated circuits to accurately sense the movement of ferrous metal targets. An integrated circuit, along with a capacitor and a bias magnet, are sealed in a probe type package which provides physical protection and mounting means. As a geartooth passes by the sensor face of a geartooth sensor, it concentrates the magnetic flux from a bias magnet. The sensor detects the change in flux level which translates into a change in the sensor output. The current sinking digital output will swing between the supply voltage and the saturation voltage of the output transistor.
An SAE Technical Paper titled "Digital Hall Effect Position/Motion Sensing of Offroad Equipment Components" by Thomas J. Gastel was presented at the International Off-Highway & Power Plant Congress and Exposition in Milwaukee, Wis. in September 1988. The paper discusses Hall effect magnetic solid state position and motion sensors. A new type of probe sensor was described. The sensor functions by sensing a change in magnetic field intensity and converts this change into an easy to use digital output voltage signal. The sensor discussed will be appropriate for use in extremely dirty and harsh environments found in off road applications. Possible uses of the sensor which are envisioned and described in this paper are traction systems, engine timing and fuel control systems, power shaft speed sensing, transmission control, wheel speed sensing, tachometer sensing and geartooth sensing. The paper includes an overview of the technology used and guidelines for incorporating the sensor in many different applications.
U.S. Pat. No. 5,055,768, which issued to Plagens on Oct. 8, 1991, describes a temperature compensator for Hall effect circuits. A resistor is formed in the same epitaxial layer of semiconductor material in which a Hall effect element is formed. The resistor is used to provide a temperature dependent voltage source which is inversely proportional to the resistance of a temperature sensitive load resistor on the Hall element output. A current mirror circuit is used to apply a current through the epitaxial layer resistor to the load resistor so that the voltage across the load resistor varies in a direct relationship with the sensitivity of the Hall element.
U.S. Pat. No. 4,734,594, which issued to Nelson on Mar. 29, 1988, discloses an offset correction for a sensor with temperature dependent sensitivity. A Hall effect device with offset compensation is provided with output terminals. The Hall effect element is formed in an epitaxial layer and the output terminals are connected to a differential current source. The sum of first and second currents produced by the source is determined by a resistor formed in the epitaxial layer in which the Hall effect element is formed and which is powered by the same electrical source as the Hall effect element so as to produce a current which tracks the current through the Hall effect element with temperature. The current through the resistor is split by a pair of trimmable temperature insensitive resistors and supplied to a pair of cross-coupled current mirrors which supply the currents to the output terminals of the Hall effect elements.
U.S. Pat. No. 4,760,285, which issued to Nelson on Jul. 26, 1988, discloses a Hall effect device with epitaxial layer resistive means for providing temperature independent sensitivity. The output signal of a Hall element is amplified by an amplifier circuit whose gain is determined by a resistor that is partially formed in the same epitaxial layer in which the Hall effect integrative circuit and Hall element are formed. A first amplifier stage is configured as a voltage to current converter and is connected through a current mirror to a second amplifier stage configured as a current to voltage converter. The current bias for the first amplifier stage is controlled by a resistor that is also partially formed in the epitaxial layer.
Geartooth sensors made in accordance with the prior art have certain disadvantages. For example, the use of a fixed offset magnitude which is added to the minimum signal value limits the flexibility of the sensor and makes it susceptible to variations in the gap between a magnetically sensitive component and the ferromagnetic teeth of a gear. On the other hand, geartooth sensors which calculate an average value of the output signal from a magnetic sensitive component are susceptible to variations in the gap between the magnetically sensitive component and the ferromagnetic teeth of a gear that can result from run out of the gear. For example, if the gap between the teeth and the magnetically sensitive component varies as a function of the angular position of the gear, possibly due to misalignment or noncircularity of the gear, the maximum output signal magnitude from the magnetically sensitive component can vary as a function of the angular position of the gear. This, in turn, changes the average value that is calculated as a function of both the minimum and maximum magnitudes of the output signal from the magnetically sensitive component. Other known geartooth sensor circuits also suffer the disadvantage of not being selectable as a function of the application of the sensor.
It would therefore be advantageous if the geartooth sensor could be provided with a circuit that enables the selection of a threshold magnitude which is not dependent on the maximum value of the signal from the magnetically sensitive component.